Abstract

Jarosites are a family of minerals with the general formula AFe3(SO4)2(OH)6
(A=H3O+, NH+
4 , K+, Na+, Rb+, Ag+, 1
2 Pb2+). Long known to earth scientists
and the mining community, jarosites provide the most studied examples of model
kagome magnets: materials where the magnetic exchange interactions are frustrated
by the kagome geometry of vertex-sharing triangles. In most jarosites this frustration
is insufficient to prevent the formation of magnetic order at low temperatures and
two ordering transitions are observed. The primary transition at 60 < TN1=K < 65
is to an umbrella spin structure, which
ops into the kagome plane at the secondary
transition, TN2 , between 45 and 55 K. The exception to this behaviour is hydronium
jarosite (A=H3O+) which instead undergoes a critical freezing transition to an
unconventional spin glass state at a much lower temperature, Tg ~ 17 K.
This thesis presents studies of the chemistry, crystallography and magnetism of
the jarosites with the general aim of relating the observed magnetic responses to their
chemistry and crystal structures. The investigations presented here concentrate on
hydronium jarosite and relate the changes to the synthesis chemistry and crystal
chemistry to the spin-glass transition temperature. A combination of powder and
single-crystal X-ray diffraction and SQUID magnetometry are used to show that the
spin-glass transition in hydronium jarosite is correlated to the degree of distortion
of the coordination around the moment bearing Fe3+ ions. Samples with the most
symmetric coordination of the magnetic Fe3+ ions feature the lowest values of Tg.
As the defining influence in spin glasses is typically thought to be disorder, this
observation is remarkable. Further, these studies show that in hydronium jarosite the
key to the spin glass transition is a uniform (translationally invariant) energy scale
that is associated with the crystallographic distortion. In so doing, they support the
proposal that the spin-glass transition in hydronium jarosite is driven by anisotropy.
Further, elemental analyses show that the Fe stoichiometry has little effect upon
the displayed magnetic properties of the jarosites. Rather, these are found to be
most sensitive to small crystallographic changes in the Fe-O coordination octahedra
associated with substitution of the A-site cation. In terms of the ratio between the
Fe-O equatorial and apical bond lengths, greater deviation from Oh symmetry is
shown to correlate with higher values for Tg and TN2 .